The driving force behind the switch from incandescent lighting to LED technology is to gain power efficiency and longer life. However, the cost of LED assemblies is much higher than incandescent and CFL lamps for equivalent lumen outputs. In theory, higher LED costs can be offset by lower power consumption and longer service life. LEDs can last for 50,000 hours or more, but that may not be the case for a complete lighting assembly.
As a designer, you’re well aware that environmental factors have a strong influence on end-product reliability. Therefore, careful attention must be paid to the type of LED driver topology used and the selection of protective devices for various locations in the circuitry.
Since Littelfuse evaluates a lot of different LED assemblies, we're finding a wide variety of approaches taken in designing LED drivers and their power converters.
In addition to a host of other performance factors, the converter topology has a profound influence on assembly size, cost, safety, and reliability. So we recommend a review of industry standards governing safety and reliability before getting into the actual design. That could be a complete blog topic itself, but my intent here is to give you an overview of the two key design areas mentioned: converter topology and circuit protection. So I'll just mention that some of the most important standards include UL 8750, which addresses fire and electrical shock issues, and standards such as Energy Star, ANSI/IEEE C.62.41, and IEC61000-4-5 that specify surge immunity requirements for LED lighting.
A typical LED assembly powered from AC mains uses a series wired string of LEDs driven by a switch-mode constant current driver. This is the case for most LED retrofit bulbs and outdoor luminaires. Certain application factors may suggest a specific type of converter topology, and there are far too many to talk about in a blog. I'll just contrast two types that highlight the typical tradeoffs to be considered. The key design features I'm concentrating on are isolated vs. non-isolated topologies.
An isolated single-stage flyback driver (top) and a non-isolated buck driver.
(Source: ON Semiconductor)
Isolated vs. non-isolated designs
Isolated topologies have a transformer that provides a degree of electrical isolation between the AC and DC sides of the driver's power converter. This is desirable from a safety perspective, but it is a significant factor in terms of size, cost, and power loss due to leakage inductance and reduced utilization of winding area. Relatively large components may be needed for power factor correction (PFC) solutions to maintain constant LED current. On the other hand, isolation may reduce the number, size, and cost of circuit protective devices needed on the secondary side.
As shown in the figure above, non-isolated topologies don't have a transformer; their circuits are simpler, and the component count is lower. These topologies are prominent in residential LED retrofit bulbs and other lower-power applications. Still, adding electrical insulation to protect users from dangerous voltages adds another form of assembly cost, as LEDs require exposed heat sinks to keep their temperature below a critical value. Excessive temperature results in loss of LED efficacy, may cause color shift, and could pose a fire hazard. These circuits are also more vulnerable to surge damage, so they typically require more robust circuit protective devices on both the AC and DC sides of the power converter.
Circuit protection requirements
All LED drivers need various devices in multiple locations to protect the circuitry from overvoltage and overcurrent events. A fuse is the most common and reliable technology to interrupt or limit current flow resulting from short circuit or overload events. It minimizes the risk of shock and fire. A small form factor is important, especially in residential LED retrofit lamps.
Outdoor LED lighting is far more susceptible to high overvoltage surge events that often result from lightning storms. These applications generally need highly robust metal oxide varistors (MOVs) to shunt energy away from the power converter. For example, the DOE's Municipal Solid State Street Lighting Consortium's model spec for LED roadway lighting calls for 20kV/10kA testing, with a total of 30 hits in different coupling modes and phase angles.
Residential assemblies can usually get by with a smaller MOV located after the fuse. Outdoor luminaires probably also need the surge protection device (SPD) ahead of the fuse, which has larger MOVs that activate before the one after the fuse does. For any residual let-through energy, a transient voltage suppression (TVS) diode can be added at the input of the converter. Selection of MOV and fuse parameters should be coordinated to ensure the fuse doesn't open during transient events. It should open only in the event of a short circuit or overload condition.
Other protective devices may also be needed in high-value LED assemblies. The overall objective is to achieve end-product reliability that matches the inherently long life of LEDs. Click here for resources to help you achieve this objective.
Usha Patel is director of Latin American sales and segment marketing for Littelfuse.
Click here to learn about Littelfuse's Speed2Design site.